1. Field of the Invention
The present invention relates to an apparatus for controlling water content of fried food such as a snack food, tempura, rice cracker by using microwave energy in combination of heated air.
2. Description of the Conventional Art
In such as a snack food and precooked food, water contained in the raw materials thereof such as fresh vegetables, noodles, shellfishes, tempura is evaporated and dried so as to preserve for a long time.
In general, the temperature of the oil in a fryer is controlled in the range of about 160.degree. C. to 190.degree. C. The food is continuously transported through the fryer to be the fried food so that the final water content of the fried food is controlled to fall in the range of 0.5% to 5% to keep the product in a dry state.
However, some raw materials of the fried food are easy to be heated and the other raw materials of the fried food are difficult to be heated. The raw materials easy to be heated can be finished to achieve the above final water content by merely frying in oil. However, it may take a long time to finish the raw materials difficult to be heated. Consequently, the food contents a lot of oil, thereby having a bad quality.
If the food is fried at higher temperature in order to shorten the time for finishing, the surface of the food changes its color and quality.
In addition, since raw materials such as fresh vegetables, shellfishes, potatoes rich in saccharide are easy to be browned, the frying time have to be as short as possible.
Accordingly, the raw materials difficult to be heated and the raw materials easy to be browned are fried so that the water content thereof is in the range where the marketability thereof is not spoiled. Thereafter, the food is dried by a heating dryer using heated air or extreme infrared radiation so as to control the water content, thereby producing a dry fried food.
In the above conventional dryer, since the fried food is heated from its outside, it takes a lot of time to heat up the core portion thereof. Accordingly, it is difficult to dry the food without spoiling the marketability thereof.
Accordingly, it has been considered that heated air is supplied to the fried food simultaneously with applying the microwave energy which affects the food to heated up from the core portion thereof.
Therefore, heated air heats the fried food from the outside and the microwave heats it from the inside so that the fried food is heated and dried uniformly.
However, the exoergic principle of the microwave energy is a frictional heat due to molecular vibration caused by the microwave applied to water contained in the fried food. Consequently, with decreasing water content, its microwave absorption gets worse, thereby being difficult to be heated. That is, it takes a long time and consume much microwave so as to cause the fried food to be in a dry state (low water containing state), thereby causing unefficiecy. In addition, the power of unavailable microwave which is not absorbed by the fried food may be a reflected power returning to the microwave oscillator, thereby causing breaking the oscillator.
On the other hand, during the heating and drying, large amount of water and oil are evaporated from the fried food. The evaporated water with oil has to be discharged to the outside, because its standing in a dry chamber deteriorates the working environment and sanitation. In addition, malodorous gas must not to be discharged to the open air in view of environment pollution.